The references listed in this specification, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.
Dental implants used to stabilize dentures or support dental crowns and bridges have been known and have been used fairly extensively in the recent past. Such prior art devices are typically comprised of three components, namely, an implant component for anchoring to the bone, a transgingival component and a separate support component. The support component usually attaches to the transgingival component which, in turn attaches to the anchoring component at about the level of the bone. An artificial tooth or bridge may then be attached to this separate support component. This support component is sometimes referred to as an abutment portion, the transgingival component is sometimes referred to as an abutment connection or the transgingival collar or the transepithelial connection and the implant is sometimes referred to as a fixture. An example of such a prior device is found in Canadian Patent No. 1,313,597. This patent describes an implant for insertion into bone through an epithelial and fibrous connective tissue layer to which a prosthesis may be attached. This implant comprises a top portion for supporting a mechanical component to which the prosthesis may be connected and a body comprising an upper bone attachment region, which tapers to a lower bone engagement region having a porous surface. The upper bone attachment region comprises a substantially non-porous but bioreactive surface and this patent teaches that this results in an upper bone attachment region which is claimed to be capable of enhancing bone attachment.
However, several problems develop with an implant of this type. In particular, the patent teaches use of a collar 14 that is adapted to be coupled to the implant 12. However the interface between the collar 14 and the implant 12 occurs at a level below the gingiva in the installed position. Further, although the patent teaches providing recesses 40 on the lower surface 42 of the collar 14 to compliment projections 32 of the implant 12 to prevent rotation between the two components, in practice this is not effective. The attachment between the collar and the implant is accomplished by means of a threaded screw identified as 46 in FIG. 1. Such a screw has a natural tendency to become loose during the vigorous stresses to which an implant of this type is subjected.
To avoid problems associated with the loosening of the threaded screw 46, practitioners have resorted to insertion of cement into the threaded portion to ensure a locked and non-loosening joint between the implant component and the support component.
However, a basic problem with this structure and method still remains. Substantial forces are exerted upon a very small region where the screw is affixed within the jaw. These forces are focused about a small region about point rather than being distributed. Using a plurality of closely set screws disadvantageously lessens the amount of material to which the implant may be affixed.
Unfortunately, screws eventually become loose, and damage to the bone into which they are affixed is permanent. Thus repeated re-tightening or insertion of new screws is limited and not practicable.
It is an object of this invention to provide a "snowshoe-like" effect wherein an implant is securely affixed becoming joined to bone at a multiplicity of points over a large region.
In the aforementioned prior art implant, unfortunately, because the interface between the collar and the implant is below the gum level, any excess cement will be squeezed out at the interface and may not be noticed by the practitioner since it is hidden from view. Such excess accumulation of cement can create irritation of the gum and the bone and can result in infection and/or implant failure. In addition, all implant systems, (fixture, abutment connection, abutment) which have this type of arrangement have a microgap between the fixture or implant and the abutment connection or the transgingival collar at the level of the bone. This microgap has been called an "sendotoxin generator" by some authorities because it is a region for potential bacterial growth.
Other prior art devices include implants with threaded exteriors, which require extensive and complicated methods for preparation of the gum and bone to accept the insert. As a result, such implants are difficult and expensive to insert and specialists most often do the surgery.
This invention provides an implant and method of fabricating such which obviates difficulties and associated problems with prior art implant systems.
An aspect of this invention relates to the use of a resorbable biocompatible material such as coral, to provide the overall implant structure.
The use of these biocompatible materials is well known to assist in the regeneration of bone defects and injuries. In 1926, DeJong observed the similarities between the powder X-ray diffraction pattern of the in vivo mineral and the hydroxyapatite (Ca.sub.5 B(OH)(PO.sub.4).sub.3, (CHA). Calcium compounds, including calcium sulfate (Nielson, 1944), calcium hydroxide (Peltier, 1957), and tricalcium phosphate (TCP) (Albee et al., 1920), have been observed to stimulate new bone growth when implanted or injected into bone cavities (Hulbert et al., 1983). These materials also exhibit good biocompatibility and compositional similarities to human bone and tooth and can serve as resorbable or non-resorbable implants depending on their degree of microporosity.
Some TCP implants are known to be readily resorbable. For example, sintered TCP plugs with pore sizes between 100-200 microns have been implanted in rats (Bhashar et al., 1971). Very rapid bone formation was reportedly observed at three days after implantation, and highly cellular tissue, consisting of osteoblastic and fibroblastic proliferation, was found within the pores. At one week, the size of the implant was reduced, and new bone formation was extensive. After two weeks, connective tissue had infiltrated throughout the ceramic. During the next four weeks, the bony material within the ceramic continued to mature. Electron micrographs indicated that within clast-like cells, ceramic could be depicted in membrane-bound vesicles. The authors concluded that TCP implants were biodegradable, via phagocytosis, the ceramic did not elicit a marked inflammatory response, and connective tissue grew rapidly within the pores. Similar results have also been reported by Cutright et al. (1972) who also implanted TCP in rat tibiae. In this study, the ceramic cavities were filled with osteoid and bone after 21 days and the TCP implant was no longer detectable after 48 days.
Larger implants in dogs are reported to elicit slower responses. Cameron et al. (1977) found that TCP implants in dog femurs were completely infiltrated with new bone by four weeks. However, after six weeks, the rate of new bone growth had slowed as the TCP was resorbed. Additionally, only 15% of a 2 cm.times.2 cm iliac TCP implant in dogs was resorbed after 18 months (Ferraro et al., 1979). Koster et al. (1976) reported the testing of the calcium phosphate formulations monocalcium phosphate, dicalcium phosphate, tricalcium phosphate, tetracalcium phosphate, and combinations consisting of 20% monocalcium phosphate and 80% of either di-, tri- or tetracalcium phosphate as implant materials in dog tibiae. These investigators tested both dense ceramics and porous ceramics with pore sizes between 800-1000 microns. They reported that tissue compatibility is dependent on the CaO/P.sub.2 O.sub.5 ratio. All materials with ratios between 2/1 and 4/1 are compatible with the optimum ratio being about 3/1 for TOP. After 10 months, Koster et al. (1977) found that tetracalcium phosphate was resorbed only to a minor extent, but that TCP demonstrated lamellar bone growth throughout its pores. Both were found to be tissue compatible. The authors stated that the 3/1 material was not as strong as the 4/1 material and suggested that TCP should be used only in low stress areas while tetracalcium phosphate could be used in high stress environments. Jarcho et al. (1976, 1977) reported the development of a process for preparing dense, polycrystalline, calcium hydroxyapatite (CHA), with the empirical formula 2 (Ca.sub.5 (PO.sub.4).sub.3 OH) or (3Ca.sub.3 (PO.sub.4).sub.2)Ca(OH).sub.2. In this study, plugs were fabricated at 100% density and implanted in dogs. No evidence of tissue inflammation occurred, and in contrast to the porous TCP implants described above, little resorption or biodegradation was observed after six months. Holmes (1979) reported that resorption did occur in porous CHA structures. These results led deGroot (1980) to suggest that all calcium phosphates are degradable (resorbable), but the rate is determined by the degree of microporosity. A dense calcium phosphate with negligible porosity would thus degrade only nominally. These results seem to be verified by Farris et al. (U.S. Pat. No. 4,673,355), who claim biocompatible materials with good properties over the range of Ca/P atomic, or molar, ratios from 0.1 to 1.34. These ratios convert to CaO/P.sub.2 O.sub.5 ratios between 0.2 and 2.68, lower than the 3.0 ratio suggested above. They suggest that the Ca/P or CaO/P.sub.2 O.sub.5 ratio is not critical for implant applications. Ca/P ratios in the range 0.1 to 2.0 probably show satisfactory biocompatibility. Capano (1987) found that a Ca/P ratio of 0.5, which corresponds to calcium metaphosphate ("CMP"), has the best biocompatibility when implanted in small animals. As the apatites are nearly identical in properties and chemical compositions to bone and tooth enamel, a considerable amount of synthetic effort has been done in this area. Patents in this area include: U.S. Pat. Nos. 4,046,858; 4,274,879; 4,330,514; 4,324,772; 4,048,300; 4,097,935; 4,207,306; and U.S. Pat. No. 3,379,541. All of these patents are incorporated herein by references. Several patents describe methods for treating apatite materials to render implantable shapes. These methods of heating and compaction under pressure in molds produce solid porous articles in various shapes. These patents include: U.S. Pat. Nos. 4,673,355; 4,308,064; 4,113,500; 4,222,128; 4,135,935; 4,149,893; and U.S. Pat. No. 3,913,229. Several patents speak to the use of laser radiation to bond apatite materials to tooth and other surfaces, for example, U.S. Pat. No. 4,673,355 and U.S. Pat. No. 4,224,072. Other patents describe the use of particulate or compacted apatite in conjunction with various compounds, filler, and cements, for example, U.S. Pat. Nos. 4,673,355; 4,230,455; 4,223,412; and U.S. Pat. No. 4,131,597. The aforementioned patents are all incorporated herein by reference. The above discussion indicates that calcium phosphates or compounds, such as CHA that are substantially TCP (Monsanto, for example, markets CHA as TCP), are useful for a variety of bioceramic applications because they are biocompatible and can be fabricated into shapes that have a desirable combination of strength, porosity, and longevity for particular sorbable and non-sorbable needs. Virtually any calcium and phosphate source can be used to prepare materials of interest.
This is explained in more detail in U.S. Pat. No. 5,639,402 issued Jun. 17, 1997 and entitled Method for fabricating artificial bone implant green parts, incorporated herein by reference.
Some more recent advances are the development of hydroxyapatite (CHA) and calcium phosphate powders that can be processed to yield porous resorbable bone facsimiles (U.S. Pat. No. 4,673,355); the development of the SLS.TM. process for directly shaping complex porous structures from thermally fusible polymer/ceramic powders without molds (U.S. Pat. No. 5,076,869); the development of low temperature infiltration and cementing techniques to prepare and replace the polymer binder with ceramic binder (U.S. Pat. No. 5,284,695); and the development of techniques for converting computed tomographic ("CT") information into three-dimensional mathematical files that can automatically guide the SLS.TM. process (Levy et al., 1992; Levy et al., 1994).
More recent work has been directed at expanding the utility of the SLS.TM. apparatus by preparing polymer-coated ceramic powders from spray dried mixtures of water, inorganic particulate, and a custom-synthesized, emulsified, nanometer-sized, polymer binder (Barlow, 1992; Vail et al., 1992). Ceramic composites made by this approach are relatively large, 10-50 microns, agglomerates of polymer-coated inorganic particles. These agglomerate powders may spread easily into uniform layers and fuse readily in the SLS.TM. machine to yield porous "green" parts that have relative densities near 50%, excellent connected internal porosity, and sufficient strengths to be easily handled and shipped. Interconnected pores in bioceramics are often difficult to achieve and are very important in fostering bone growth and for preparing metal matrix/ceramic parts, artificial hips. Polymethyl methacrylate (PMMA) has also been used to form green composites with alumina and with silica/zircon (U.S. Pat. No. 5,284,695). In this process, an appropriate ceramic silicate colloid is used to infiltrate the connected pores of the polymer-bound green part, the colloid is solidified below the fusion temperature of the binder to maintain part geometry, the binder is then thermally removed and the part fired at typically 1000.degree. C. to form porous, all ceramic parts that are suitable for use as cores and molds for metal castings. Such parts typically have only a 1% linear shrinkage, relative to the green state. Their strengths and porosities can be adjusted by additional infiltration and firing treatments. Lagow and co-workers have recently described the chemical synthesis of high strength CHA (U.S. Pat. No. 4,673,355) and long-chain calcium polyphosphate bioceramic powders ("CPB") (Capano, 1987; Nelson et al., 1993). CPB powder is a pure calcium phosphate material with condensed phosphate chains (as shown below) with degrees of polymerization often greater than 120. These materials produce sintered materials that have compressive strengths greater than 200,000 psi and flexural strengths in excess of 20,000 psi. These strengths are about twice that of porcelain used to make dental crowns. Using the Lagow CHA material, Lagow and Friedman have recently completed the first successful, year duration, mandible implant in a canine. Work with CPB implants has demonstrated by electron microscopy backscattering that new bone growth occupied nearly 55% of the volume of a CPB implant in the alveolar (tooth bearing) ridge of a dog, after only four months (Nelson et al., 1993).
It is an object of this invention to provide a dental implant that overcomes many of the disadvantages of known implants.
It is an object of this invention to provide an implant that will be substantially resorbed and replaced with bone.